1. Field of the Invention
The present invention relates to a lens control device and an image sensing device using such a lens control device.
2. Description of Related Art
Conventionally, there is proposed an image sensing device that has a camera shake correction function of an optical lens shift type (hereinafter simply referred to as the “camera shake correction function”). In the camera shake correction function, when a camera shake occurs, the target position of the lens is calculated such that the blurring of an image is reduced. Then, the lens is shifted to the target position, and thus the camera shake correction is achieved.
In the image sensing device having the camera shake correction function, in general, the lens is movably supported by a spring to a lens unit. The lens is also shifted by a voice coil motor (VCM) to the target position. The VCM is designed to drive (shift) the lens according to a motor current that has been fed.
In the image sensing device having the conventional camera shake correction function, control is performed such that the position of the lens basically coincides with a reference position (for example, the center position of a range of movement) that is fixed within the range of movement of the lens. In other words, when the camera shake correction function is disabled or when no camera shake occurs, the posture of the lens is held in the reference position. When the camera shake correction is performed, the lens is shifted with the reference position being the center of the movement.
FIG. 7 is a diagram schematically showing an example of the support form of the lens. In FIG. 7, the vertical direction represents the direction in which the lens is moved (here, only one direction is considered), and O point (a balance position when the weight of the lens is ignored) is the reference position. In the left side of the FIG. 7, a state where the drive force of the lens exerted by the VCM is zero is shown; in the right side of FIG. 7, a state where the lens is being shifted by the VCM to the reference position is shown.
As shown in the left side of FIG. 7, when the drive force of the lens exerted by the VCM is zero, the lens 51 is present in a position (hereinafter also referred to as a balance position) where the weight of the lens 51 balances the elastic force of springs 52 and 53. Specifically, the position of the lens 51 is a position that is obtained by displacing the reference position by I=mg sin θ/2k, where m represents the weight of the lens, g represents the gravitational acceleration, θ represents an angle formed by the direction of the movement and the direction of gravity and k represents a spring constant.
On the other hand, as shown in the right side of FIG. 7, in order to hold the lens 51 in the reference position that has been displaced from the balance position, it is necessary to continuously apply the corresponding force to the lens 51. Specifically, in order to hold the lens 51 in the reference position, a constant drive force exerted by the VCM is needed. As the drive force that the VCM is required to exert is increased, a motor current that needs to be fed to the VCM is increased.
For the reason described above, when the lens is held in the reference position, as compared with a case where the lens is held in the balance position, a large motor current is needed. Likewise, even when the lens is shifted with the reference position being the center of the movement, as compared with a case where the lens is shifted with the balance position being the center, a large motor current is needed.
In terms of power consumption saving and the like, the motor current necessary for controlling the position of the lens is preferably minimized. In particular, in a mobile image sensing device that is used in a mobile telephone or the like, since a power supply capacity is often limited, it is extremely required to reduce the motor current.
Under the above conditions, the position in which the lens is held and the center of the shift movement are preferably present near the balance position. The change of orientation of the image sensing device (for example, caused by the holding of the image sensing device upside down by a user) or the like changes the direction in which the weight of the lens acts (the direction relative to the orientation of the image sensing device), and the balance position is changed accordingly. Hence, in order to keep the position in which the lens is held and the center of the shift movement near the balance position, it is necessary that the position in which the lens is held and the center of the shift movement can follow the change of the balance position.